update date in API docs

doc/fluidsynth-v11-devdoc.txt

@@ -9,7 +9,7 @@
 \author Tom Moebert
 \author Copyright © 2003-2018 Peter Hanappe, Conrad Berhörster, Antoine Schmitt, Pedro López-Cabanillas, Josh Green, David Henningsson, Tom Moebert
 \version Revision 1.1.11
-\date 2018-02-24
+\date 2018-05-04
 
 All the source code examples in this document are in the public domain; you can use them as you please. This document is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/3.0/ . The FluidSynth library is distributed under the GNU Lesser General Public License. A copy of the GNU Lesser General Public License is contained in the FluidSynth package; if not, visit http://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt or write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.